High molecular weight water-soluble anionic polymers are useful in a number of applications e.g. the flocculation of suspended solids, recovery of minerals from mining operations, coal refuse dewatering, papermaking, paper sludge deinking, enhanced oil recovery, wastewater treatment, soil conditioning, etc. In many cases, the anionic polyelectrolytes are supplied to the user in the form of substantially dry polymer granules. The granules may be manufactured by the polymerization of water-soluble monomers in water to form a water-soluble polymer solution, followed by dehydration and grinding to form water-soluble polymer granules.
Another means for isolating the polymer from the polymer solution is to precipitate the polymer by mixing the polymer solution with an organic solvent e.g. acetone or methanol that is a non-solvent for the polymer, then isolating the polymer by evaporation or filtration. However, in many cases, this method is inconvenient, expensive and dangerous because of the problem of handling large amounts of flammable organic solvent.
Water-soluble anionic polymers may also be supplied in the form of a water-in-oil emulsion or microemulsion, wherein the polymer solution droplets are isolated from each other by the continuous oil phase. The polymer emulsions may be utilized directly in the desired application, or by diluting into water in the presence of a "breaker" surfactant. Although this mode of supply is convenient and may avoid the need for dehydration, the oil may be expensive and is often flammable; in addition, the oil may also present a secondary pollution problem. Alternatively, the emulsion may be precipitated into an organic liquid that is a solvent for the water and oil, but a non-solvent for the polymer, followed by isolation and drying to recover the substantially dry polymer. However, these precipitation methods may be disadvantageous for the same reasons mentioned above.
Processes for preparing water-soluble polymers in the form of unswollen, hard, nontacky granules are described in U.S. Pat. No. 3,336,270. The water-soluble polymers were prepared by dissolving acrylamide-type monomers in tertiary butanol-water mixtures and allowing the monomer to polymerize to give polymers which precipitated out of the tertiary butanol-water mixture.
A first water-soluble polymer may also be dispersed in the presence of a second water-soluble polymer to form aqueous polymer dispersions, as taught in U.S. Pat. Nos. 4,380,600 and 5,403,883. Since the two polymers do not dissolve each other, the first water-soluble polymer reportedly forms small globules which disperse in the solution of the second water-soluble polymer. Optionally, salt may be added to improve the flowability.
U.S. Pat. No. 3,891,607 disclosed thermoreversible coacervates that were produced by copolymerizing from 30 to 50 mole percent of acrylic acid and from 70 to 30 mole percent of acrylamide in aqueous solution, lowering the pH to below 3.3 and adjusting the temperature to below the coacervate transition temperature.
U.S. Pat. No. 3,658,772 describes a process for the copolymerization of acrylic acid in a salt solution containing 0.1 to 10 percent salt, by weight, based on total weight, to form a polymerizate in the form of a fluid suspension of disperse, solid-polymer particles. Hereinbelow, all concentrations, unless otherwise noted, are expressed as weight percent of total weight. Significantly, the pH of the polymerization was in the range of 1 to 3.2, and it was reported that increasing the pH to 4 and above resulted in non-fluid, gel polymerizates, apparently because of the increased solubility of the salt form of acrylic acid at higher pH. In U.S. Pat. No. 3,493,500; the pH range was increased to as high as 4 by including a cationic water-soluble polymer in the formulation in an amount of about 0.03 to 0.2 part per part by weight of acrylic acid polymer solids. However, in neither case were fluid suspensions obtained at pH values higher than 4.
Japanese Patent Publication No. 14907/1971 discloses a method for the copolymerization of acrylic acid and acrylamide in salt solutions to form a flowable polymerizate. The copolymerization was conducted at a pH of 1 to 4 in the presence of 0.1-60 wt % inorganic salt. In several systems containing 90/10 acrylic acid and acrylamide and 50/50 acrylic acid and acrylamide, if the pH of the polymerization system was increased to 4 or more, nonflowable gel polymerizates were produced. The homopolymer of acrylic acid could be manufactured as a "suspended compound" at a pH of 4 or slightly higher.
Aqueous dispersions of anionic polymers which are precipitated in salt solutions at low pH are generally utilized by diluting the dispersion into water so that the salt concentration is greatly reduced. At low salt concentrations, the anionic polymers become more soluble and hence dissolve. However, the rate of dissolution tends to be a function of pH, so that if the dispersions are diluted into acidic water, the polymer dissolves at a disadvantageously slow rate, frequently necessitating addition of base to raise the pH and increase the dissolution rate. Therefore, for practical reasons, it is desirable for the pH of the polymer dispersion to be higher than 4 so that pH adjustment of the dilution water is unnecessary.
The effect of salts on the solubility of various substances in aqueous solution is well discussed in the scientific literature e.g., Kim D. Collins and Michael W. Washabaugh, Q. Rev. Biophys., Vol. 18(4) pp. 323-422, 1985. "Kosmotropic" salts tend to reduce the solubility of substances in aqueous solution. There are numerous means known to those skilled in the art for determining whether a particular salt is kosmotropic. Representative salts which contain anions such as sulfate, fluoride, phosphate, acetate, citrate, tartrate and hydrogenphosphate are kosmotropic. Some salts are more kosmotropic than others, based on the well known "Hofmeister series" principles.
The use of salts to precipitate anionic polymers is also taught in EP 183 466 B1. This invention provides a method of obtaining a dispersion of a water-soluble polymer by dissolving a monomer in an aqueous salt solution and conducting polymerization while depositing the polymer as fine particles in the presence of a dispersant. The aqueous salt solution is required to dissolve the monomer and precipitate the polymer. As the dispersant, a polymer electrolyte and/or a polymer soluble in an aqueous salt solution is/are effective. Where the deposited polymer is an anionic or cationic polymer electrolyte, the polymer electrolyte used as the dispersant is required to have charges of the same kind as the deposited polymer. Representative salts include sodium sulfate, ammonium sulfate, and other strongly kosmotropic salts. For polymers whose anionicity is derived from the presence of sulfonate groups e.g polymers and copolymers of poly(2-acrylamido-2-methylpropanesulfonate), herein poly(AMMPS), the polymer is difficult to precipitate even at low pH and high levels of kosmotropic salt.
The precipitation of anionic polymers by cationic organic salts, e.g. surfactants, is well known. A review by E. D. Goddard (Colloids and Surfaces, Vol. 19 pp 301-329, 1986), is hereby incorporated herein by reference. The precipitation phenomenon is reportedly controlled by the relative concentrations of the anionic polymer and cationic organic salt, as well as by the size of the organic portion of the anionic organic salt and by the type of polymer. Anionic polymer precipitation tends to occur when the oppositely charged cationic organic salt binds to the polymer and neutralizes the charge. The prevailing view has been that the addition of salt weakens the binding, making precipitation more difficult.
For instance, the effect of added sodium chloride is discussed on p. 313 of the review by E. D. Goddard, cited above, wherein the author states that "adding salt . . . substantially reduces the affinity of binding as seen by a steady increase in the concentration [of surfactant] at which binding commences . . . " A similar view was advanced by Y. Li and P. Dubin, in "Structure and Flow in Surfactant Solutions, ACS Symposium Series 578, American Chemical Society, 1994, at p. 328 where the authors state: "In order to avoid precipitation in mixtures of strong polyelectrolytes with oppositely charged [surfactant] micelles, the binding strength . . . must be reduced. Practically, several ways could be used to attenuate the strong electrostatic interaction between the polyelectrolyte and oppositely charged surfactant, such as . . . addition of salt."
Surprisingly, and contrary to the teachings cited above, it has now been found that the precipitation of many typical water-soluble anionic polymers by cationic organic salts in aqueous solution can be greatly enhanced by the addition of kosmotropic salts. Significantly, these polymers remain precipitated even at pH greater than 4. Therefore, in accordance with our invention, compositions comprised of water, at least one cationic organic salt, at least one kosmotropic salt, and at least one precipitated anionic water-soluble polymer are provided. Further, processes for precipitating water-soluble anionic polymers in compositions comprising water and one or more cationic organic salts, and one or more kosmotropic salts, are also embodied in the instant invention. Compositions in which the precipitated anionic polymer is dispersed in the form of small droplets so as to produce a polymer dispersion are preferred. These polymer dispersions remain flowable even at pH greater than 4. These polymer dispersions may be stabilized by a dispersant, which may be a water-soluble polymer, and the precipitated anionic polymer is preferably formed by polymerization of monomers in the salt solution, optionally in the presence of a dispersant.